Modern direct injection gasoline engines require fuel injectors to operate under extreme conditions of temperature and pressure and with high fuel pressures. Furthermore, the fuel injector must open and close very rapidly in order to provide multi-pulse injection cycles required for fuel efficiency and low emissions.
Current high pressure direct injection fuel injectors either use inwardly opening valves (nozzle type or multi-hole director) in conjunction with solenoid actuation or outwardly opening valves using piezo-electric actuation. The outwardly opening piezo-electric actuated injector has demonstrated the highest potential for reducing fuel consumption, but the cost of the piezo-stack and driver is prohibitive for high volume applications.
Known outwardly opening piezo-electric actuated fuel injectors generally comprise a valve body having a tip portion defining a spray aperture, a pintle or valve stem extending within the tip portion for axial movement between an extended and a retracted position, the pintle having an external head engageable with a valve seat of the spray aperture to close the spray aperture when the pintle is in its retracted position, a return spring biasing the pintle towards its retracted position, an actuating means in the form of a piezo-stack, acting upon the pintle to urge the pintle to its extended position when the piezo-stack is energised.
The piezo-stack can provide a high opening force to overcome the strong return spring required to hold the valve closed and the high hydraulic forces generated during the high pressure operation of the injector. The piezo-stack also provides rapid valve opening and can achieve a variable valve lift. However, piezo-electric fuel injectors are very costly to produce compared to solenoid actuated injectors and require complex and costly control systems for operation of the piezo-stack.
By contrast, solenoid actuated fuel injectors are much cheaper to produce. However, known solenoid actuated fuel injectors cannot provide the same level of performance as piezo-electric actuated devices, mainly due to the lower opening force achievable by electromagnetic solenoid actuators and the slower rise of force over time.
A particular problem with known outwardly opening solenoid actuated fuel injectors when operated at high speed is valve bounce. When closing the injector at high speed, the impact of the pintle head against the valve seat can be substantial due to the large mass of the armature connected to the pintle and the force exerted on the pintle by the return spring. Due to the elasticity of the valve surfaces and the pintle stem, the pintle head tends to rebound from the valve seat, causing the injector to re-open. Such valve bounce can cause one or more unmetered after injections of fuel delivery after injector closing. This problem is particularly acute in high pressure applications.
Known outwardly opening solenoid actuated fuel injectors utilise squeeze film damping to attempt to eliminate valve bounce by carefully controlling the air gap between the armature and the facing surfaces above and below the armature when the pintle is in its retracted and extended positions. Such gaps are required to be controlled to around 20 μm or less with slight variations leading to substantial variations in squeeze damping effect. Manufacturing and adjusting such air gaps has proven to be very expensive and difficult to control, with additional problems of durability and performance, particularly in relation to differential thermal expansion of different parts of the injector during use. Squeeze film damping forces are essentially proportional to the cube of the distance between squeeze damping surfaces and their relative velocity. Due to this highly non-linear nature, squeeze damping gaps need to be very well controlled in order to limit part by part variations in a mass produced injector. GB1197738 discloses a known injector disclosing an armature decoupled from the valve pintle.